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Dyno versus Road Power and
Torque (HP/NM)
Due to today's exploding fuel prices and focus on global warming,
car manufactures move into smaller and smaller displacement
engines to reduce average fuel consumption. They add turbos in order not
to lack car performance and gain back power and even
better torque.
It's fine, but we don’t save much on fuel if we really use the power
a lot. Anyway…
Historical cars have been tested
on Dyno ‘in fourth gear’ to describe the engine performance in terms
of power and torque. The majority focus on the maximum power
and have a small attention to the torque curve.
For dynamic street use with interval acceleration (overtaking) in the different
gears, the torque curve should be the key focus. What does the torque
curve look like over the RPM/speed range in the different gears?
Dyno test works pretty
consistent for natural
aspirated (N.A.) and fuel injection engines. Dynos have different
dynamic load, but that won’t make a big difference on an N.A. engine.
Most tests are made in the 4. gear for a 5. gear car. This is fine
for comparison of any improvements.
The situation is much different
for Turbo charged engines operating to high boost (MAP-Manifold
Absolute Pressure).
Two things will affect the power and torque curve.
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At low to
mid RPM the load of the engine will affect the power and torque
figures
-
Maximum
power will be affected of the inlet temperature and therefore
dependent on how much air is blown in front of the car during a test
Ref 1.
With more load on the wheels the engine will burn more fuel. Exhaust
flow and heat drop will turn the turbine and the compressor side will
produce more air and therefore the engine will produce more power.
Different Dyno dynamic load or gear will give different results.
Ref 2.
The power will decrease with higher inlet temperature (less oxygen
into the engine). A car on the road will typically get better air
cooling and therefore produce more power.
Turbo engine Power and Torque
‘Dyno versus Road’
The next picture shows Power and
Torque curves for a small Turbo engine (GT 2056) measured on ‘Dyno 4. gear’ and
derived from road data in 4. and 5. gear.
As you can see the torque band is much wider in the RPM range and
the torque is higher in 4. and 5. gear on the road.
The Dyno shows a peak torque of
356 NM at
4000 RPM.
On the Road in 5. gear torque is 395NM
at 3000 RPM and more than 350NM from 2700-5000 RPM. The peak
power also goes up from 230HP (171.5kW) to 255 HP (190kW) probably due to better cooling on the
road.
Performance ‘Dyno versus
Road’
To evaluate the different power
curves, I have done some interval speed calculation examples and
compare with real road measurements.
Let's take accelerations from
80-120 km/h (50-75 mph)
In 4. gear the RPM range is
2800-4200 and in 5. gear 2296-3445 RPM.
TM MK1
|
80-120 km/h |
Based on Dyno |
Based on Road |
Measured on Road |
|
4. gear |
3.56 sec. |
3.23 sec. |
3.12 sec. |
|
5. gear |
6.00 sec |
4.60 sec. |
4.70 sec. |
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